Biodegradable resins are known in the state of the art, being interesting because of their environmental advantages and versatility. As part of the family of biodegradable polyesters, poly(lactic acid) or PLA is the most attractive due to its properties and ease of production. Poly(lactic acid) is an aliphatic polyester that contains or may contain two enantiomers, poly(D-lactic acid) and/or poly(L-lactic acid) as components of the polymer chain and depending on the preparation technique. PLA has many useful properties such as its low flammability, high resistance to ultraviolet rays and biocompatibility. PLA is also biologically degradable, which makes it an attractive polymeric material for the plastics industry.
However, the possible uses of PLA are limited because of its high fragility, its low elongation to breakage and inadequate mechanical properties for certain applications requiring rigidity. Furthermore, PLA has the property of presenting a relatively low barrier to oxygen and to water vapour compared to other conventional polymeric materials. Therefore there is a need to provide a PLA with improved barrier properties together with greater strength to thermal deformation and with improved physico-mechanical properties compared to commercial pure PLA grades. In this direction, many attempts have been made to develop new plastics based on PLA with improved properties. However, the solutions found to date are still unsatisfactory because in addition to making the product more expensive the improvement always requires an addition or mixture with another polymer and/or additive.
An attempt described in the state of the art for improving the mechanical properties of pure PLA was to mix PLA with other thermoplastic materials (see for example, the journal of Polymer Degradation and Stability, Vol. 95, 262-269 (2010), V. Berthé et al, “extrusion of mixtures of poly(L-lactic acid) with poly(ε-caprolactone)” the improving the water vapour barrier properties. However, the use of mixtures generally implies disadvantages or is subject to limitations of use. Mixtures of PLA with other thermoplastics must be prepared at high temperatures to ensure good homogenisation of the mixture so, on the one hand this limits the type of thermoplastic material to be combined with PLA and, on the other hand, requires temperature control because PLA starts to degrade at temperatures above 180° C. Furthermore, the majority of biodegradable polymers on the market are immiscible with PLA, which implies the use of compatibilisers, and this is a limitation to take into account in the preparation of mixtures of PLA with other thermoplastics.
Another of the ways for improving PLA properties has been through the development of nanocomposites, with the aim of improving both the mechanical properties and the barrier properties to oxygen and water vapour of pure PLA. There are studies based on the use of nanoparticles such as nano calcium carbonate (see, for example, Composites part B, Vol 45, 1646-1650 (2013), J-Z Liang et al, “Crystalline properties of poly (L-lactic acid) composites filled with nanometer calcium carbonate). This method only slightly improves the mechanical properties of traction resistance of PLA but does not improve the barrier properties.
A method developed for improving the barrier properties that can be cited is that described by the authors of the international patent application WO2012017025 where a process is described for obtaining a PLA nanocomposite with an organically modified laminar phyllosilicate that has improved barrier properties against oxygen and water vapour compared to pure PLA.
Also, international patent application WO201130766 describes a process for obtaining stereocomplexed poly(lactic acid) crystals. The PLA obtained by this process differs from that of conventional poly(lactic acid) in that it comprises a higher content of stereocomplexed PLA crystals. This composition has a high melting point and is useful in making a modelled body, synthetic fibre, porous body or an ionic conductor.
With respect to nanostructured biodegradable materials by diblock copolymers, the state of the art is limited to the development of diblock copolymers in the field of biomedical applications, for example, European patent EP2364127 describes an eye implant based on a biodegradable membrane configured for a specific region of the eye. This eye implant in the form of a flexible membrane contains an active ingredient that is implanted between the intraocular lens and the surface of the posterior capsule of the eye. Its aim is to inhibit migration of epithelial cells after cataract surgery.
Also, the international application WO201221108 describes a biodegradable eye implant with a controlled release drug and a method of treatment of ocular inflammatory diseases. This implant degrades by simple hydrolysis in the body and comprises a first layer containing a biodegradable polymer and a drug dispersed or dissolved in this polymer. A multiple layer biodegradable eye implant is also described, with a first layer that is described to be on top and a second layer that comprises in turn a second biodegradable polymer arranged adjacent to the first layer.
Finally, international patent application WO201252186 describes block copolymers containing one block with one or more L- or D-lactide monomer units and one block with one or more monomer units other than L- or D-lactide. This document refers to the use of this copolymer for the preparation of a plastic article that has higher resistance to thermal deformation than pure PLA with an identical number of L- or D-lactide monomer units. This copolymer contains a block of poly(methyl methacrylate) (PMMA). The block with one or more monomer units other than L- or D-lactide is selected from styrene, acrylate, particularly MMA, olefins, particularly propylene and its derivatives; this fact implies that this block copolymer loses its biodegradability.
The synthesis of copolymers with different molar ratios has also been investigated in the literature and their crystallinity, thermal properties and morphology have been studied. The techniques most often employed for obtaining these materials have been “Spin coating” or “Solvent coating” but materials with macroscopic properties that can have commercial applications have not been provided to date.
Therefore, there is still the need to provide polymeric materials based on PLA that have improved physico-chemical properties and, especially, materials in which the barrier properties against oxygen and water vapour can be modulated depending on the needs of the application. There is still no polymeric material in the state of the art that has the properties of selective permeability to oxygen and water vapour modulated to application requirements and that has not been obtained through the addition of a nano-reinforcement. To obtain this improvement in the permeability properties without detriment to other properties such as transparency or other decisive properties for use is without doubt a problem still to be resolved.